Allow to use issue templates on service desk

What does this MR do?

Users will be able to select issue templates to be applied on service desk issues created by email.

The template is found by file path so we store it on a separate table called service_desk_settings. In case the template
is not found a note is created on the issue with a warning.

related to #2030 (closed)

Frontend TODO - Will be done on a separate merge request

• In project service desk settings form, send params[:issue_template_key] from the template selector to Projects::ServiceDeskController#update to be persisted in backend (the same template selector of new issue view can be used)
• Show a warning on project service desk settings view if issue template file is missing, backend is returning a parameter template_file_missing as true in this case.
• After frontend is done, update documentation screenshot containing the issue template selector and add explanation in doc/user/project/service_desk.md

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team